1. Field of the Invention
The present invention relates to a power supply device, and especially relates to a power supply device with low power dissipation and low fabrication cost, which utilizes a separation device to reduce voltage applied to a switch-control circuit.
2. Description of the Related Art
Recently, most computer products and their peripherals all utilize a so-called switching power supply whose center uses a pulse-width modulation control integrated circuit (IC) as a power supply control circuit. Generally speaking, this kind of pulse-width modulation control IC, for example, 3842 series and 3844 series, will not start until sufficient voltage or current is applied to it. So, it needs a startup circuit to put the IC into normal operation. After the IC is started, the energy needed by subsequent operations is supplied by a minor power supply (which is generated after the IC is started). Thereafter, the starting circuit becomes useless but nevertheless keeps on consuming power.
FIG. 1 shows a circuit diagram of a conventional switching power supply; a stabilizing control circuit is negligible. As shown in FIG. 1, a rectifier 1, such as a bridge rectifier, transforms an alternating-current (AC) voltage into a direct-current (DC) voltage; and a rectifying capacitor 2 smoothes the rectified DC voltage. The DC voltage flows through a starting resistor 3 to charge a starting capacitor 4 to start a power supply control circuit 5 (for example, a 3842 controller). After power supply control circuit 5 is started, a high-frequency pulse signal generated from the power supply control circuit 5 is transmitted to a gate of a control transistor 6 (for example, an NMOS). Then, the control transistor 6 begins a rapid switching operation. A power transformer 7 comprises: a primary winding 7a, a secondary winding 7b, and a tertiary winding 7c. The primary winding 7a is coupled to a drain of the NMOS transistor 6, and the secondary winding 7b and the tertiary winding 7c are respectively induced by the rapid switching operation of the NMOS transistor 6 and output a high-frequency pulse voltage. The induced high-frequency pulse voltage from the secondary winding 7b is rectified by an output diode 9 and smoothed by an output capacitor 10 to act as an output power source of the power supply. The induced high-frequency pulse voltage from the tertiary winding 7c is rectified by a auxiliary diode 8 and smoothed by the starting capacitor 4 to supply operational needs of the power supply control circuit 5.
The operations of the power supply are described below. When an AC voltage is fed to the power supply, the AC voltage is transformed into a DC voltage via the rectifier 1 and the rectifying capacitor 2. Then, the DC voltage flows through the starting resistor 3 to charge the starting capacitor 4. Along with an elevation of the DC voltage at terminal A, the DC voltage level at terminal B is also raised. When the voltage at terminal B reaches a specific level (for example, 16 V, in the case of a controller being a 3842 controller), the power supply control circuit 5 starts to output a high pulse signal (then, the voltage at terminal B will drop to 10 V.about.16 V). The control transistor 6 receives the high-frequency signal and begins a switching operation to transfer energy to the secondary winding 7b and the tertiary winding 7c. The high-frequency pulse voltage generated from the secondary winding 7b is rectified by an output diode 9 and smoothed by an output capacitor 10 to supply other circuits (not shown). The high-frequency pulse voltage generated from the tertiary winding 7c is rectified by an auxiliary diode 8 and smoothed by the starting capacitor 4 to supply the power supply control circuit 5.
That is, in the beginning when an AC voltage is fed to the power supply, the DC voltage which starts the power control circuit 5, flows through the starting resistor 3 to charge the starting capacitor 4. After the control transistor 6 begins the switching operation, a working voltage of the power supply control circuit 5 is supplied by the tertiary winding 7c of the power transformer 7. However, additional power dissipation occurs when current keeps flowing through the starting resistor 3. The power dissipation of the starting resistor 3 can be calculated as subsequent steps (in the case of the power supply control circuit being a 3842 controller).
Generally speaking, AC power supply by which computers and computer peripherals operate is between 90 V.about.160 V. When the AC input voltage is 90 V, i.e. the minimum input voltage, the DC voltage from rectifier 1 and rectifying capacitor 2 is about 90 V.times.1.414=126.7 V. A minimum starting current of a 3842 controller is 1 mA, so a maximum value of the starting resistor 3 is:(DC voltage-starting voltage)/1 mA=(127.26 V-16 V)/1 mA=111260.OMEGA.. When the AC input voltage is 264 V, i.e. the maximum input voltage, the DC voltage from the rectifier 1 and the rectifying capacitor 2 is about 264 V.times.1.414=373.296 V. After startup, a 3842 controller only needs a working voltage of 10 V. The power dissipation of the starting resistor 3 is (DC voltage-working voltage) .sup.2 /(the resistor of the starting resistor 3)=(373.296 V-10 V).sup.2 /111260.OMEGA.=1.18 W. Since a power saving function, a required function of computers and computer peripherals, has a power dissipation of 5 W.about.8 W or less under off-mode, a power dissipation of 1.18 W is not negligible.
In order to reduce power consumed by the starting resistor 3, one of the prior art techniques shown in FIG. 2 has been disclosed in U.S. patent application Ser. No. 5,581,453. In FIG. 2, a switching circuit 11 is connected in series to a starting resistor 3 and a power supply control circuit 5. A switch-control circuit 12 controls the turning on and off of the switching circuit 11. Before the power supply control circuit 5 starts, the switching circuit 11 keeps turning on such that a DC current charges a starting capacitor 4 through a starting resistor 3 and provides a voltage level to start the power supply control circuit 5. After the power supply control circuit 5 starts, a rectified voltage of a tertiary winding 7c can raise a voltage level at terminal B to activate the switch-control circuit 12 and turn off the switching circuit 11. The starting resistor 3 can prevent additional power consumption after the power supply control circuit 5 starts.
According to the method shown in FIG. 2, a 3842 controller, the most common power supply control circuit, needs a startup voltage of 16 V, so the active voltage of the switch-control circuit 12 should be at least 16 V. If the activation of a switch-control circuit 12 opens the switching circuit 11 before the 3842 controller 5 completes startup, no DC current flows through the starting resistor 3 and the starting capacitor 4 is not charged. In this manner, the 3842 controller 5 will have difficulty starting. After the 3842 controller 5 starts, an output voltage from a tertiary winding 7c produces an activation of the switch-control circuit 12. The output voltage of the tertiary winding 7c should be raised. As a result, the number of windings in the tertiary winding 7c should be increased. However, various problems occur if the number of windings of the tertiary winding 7c is increased. For example, the fabrication cost and the power consumption of the transformer 7 will be increased. Also, raising the output voltage of the tertiary winding 7c places greater demands on the auxiliary diode 8, the starting capacitor 4, and the control transistor 6. As a result, the cost is increased. The problems become more severe when the power supply device has a higher starting voltage.
On the other hand, after a power supply control circuit has started, the voltage level of the output pulse signal is proportional to the voltage level at the starting terminal. After the starting of the 3842 controller, the voltage level at the starting terminal drops to between 10 V and 16 V. However, in the prior art shown in FIG. 2, in order to ensure a normal operation of the switch-control circuit 12, the voltage of the starting terminal of the 3842 controller should be minimally maintained above 16 V, higher than the voltage at the starting terminal of the 3842 controller. The voltage level of the output pulse signal is also raised. Selection of the MOS transistor requires a MOS transistor having a higher gate-source voltage (V.sub.gs). The cost and the power consumption are also raised, especially when a power supply device having a high starting voltage is utilized.
In order to overcome disadvantages in the prior art, the present invention provides a power supply with low power consumption. In the present invention, a separation device is utilized to separate the starting voltage from the output voltage generated from the tertiary winding supplying a working voltage of a switch-control circuit. Even when the working voltage of the switch-control circuit is not raised, the switch-control circuit still operates normally. After a power supply device starts, the voltage at the starting terminal will not be affected by the output voltage from the tertiary winding. So a lower working voltage can maintain normal operation of the power supply control circuit. Hence, the fabrication cost and the power consumption are efficiently reduced.